In a code division multiple access (CDMA) system, a transmitter spreads an information sequence to be transmitted in the same frequency band by using different spread codes channel by channel and transmits it as a transmission signal, and a receiver receives the transmission signal as a reception signal and despreads the reception signal with the same spread code as used in transmission to acquire the information sequence.
In the most basic direct spread CDMA system, every information sequence to be transmitted in the same frequency band is spread by different spread codes and codes which have both excellent auto-correlation and cross-correlation are normally used as spread codes.
When a mobile unit moves over areas covered by a plurality of base stations in case where the CDMA system is adapted to mobile communication, a process called hand-over which switches the base station that retains the mobile unit is carried out. Hand-over in the CDMA system generally takes a scheme of receiving transmission signals from both the base station of a handing-over side and the base station of a handed-over side and sequentially shifting the reception ratio from the base station of the handing-over side to the base station of the handed-over side, whereby hand-over without interruption of a received information sequence is executed.
By the way, a mobile unit receives a synthesized wave of a plurality of frequency- and time-multiplexed transmission signals as a reception signal, though individual spread codes are different, and acquires an information sequence addressed to itself by performing despreading with a predetermined spread code. If a reception circuit is designed to have a plurality of despreading circuits, therefore, the reception circuit can receive a plurality of information sequences.
In this case, if the transmission timing differs base station by base station, a mobile unit needs to match the timings of the transmission signals from both base stations. As a result, the amount of information to be stored increases. That is, the size of a memory or the like for storing information increases, which is inconvenient for a mobile unit that needs to be made compact. It is therefore desirable that the transmission timings of both base stations should perfectly coincide with each other.
To match the transmission timings, base stations generally transmit transmission signals at a specific timing on the absolute time using GPS and it is necessary to define the absolute time at the transmission antenna end of each base station in order to completely match the transmission timings of the individual base stations with one other.
However, not every base station can be sited under the same site conditions, and the feeder line length to the antenna from the transmission device of a base station often differs from one base station to another.
To match the heads of the periods of spread codes, the heads of the spread codes need to be adjustable to an arbitrary timing. In general, this kind of timing adjustment uses a memory, such as a FIFO memory, and spread codes for a predetermined delay are stored in the memory, after which spread signals are output from the memory. That is, a circuit, such as a FIFO memory, for sampling information for one period of a spread code with the resolution for adjustment with predetermined precision and storing it is needed.
To ensure adjustment with high precision by increasing the resolution, however, the circuit scale of a base station increases, leading to a cost increase. In case where the period of a spread code is 64 octets, for example, the capacity of a FIFO memory or the like needed increases to 256 octets from 64 octets in order to increase the resolution quadruple.
The invention aims at providing a transmission circuit for CDMA communication which can adjust the transmission timing while reducing the circuit scale.